Photovoltaic generator circuits can be used to control the operation of switching devices, such as, for example, relays and transistors. A solid state integrated photovoltaic generator circuit is disclosed in, for example, U.S. Pat. No. 5,549,762 to Cantarini. The circuit typically includes an array of photodiodes connected in series with one another, and a switching device, such as, for example, a transistor, connected in parallel with the photodiode array.
A typical fabrication process for photovoltaic generator circuits involves numerous mask and etch steps which define within the silicon substrate distinct p-type (+) and n-type (-) regions which are necessary for semiconductor operation. For example, a single crystal silicon wafer of either the n- or p-type is first oxidized and then overlaid with a layer of poly-crystal silicon. The poly-crystal silicon layer is then doped with a complementary dopant to define a resistive element within the circuit. The doped poly-crystal silicon layer is then selectively masked and etched, and an insulating silicon dioxide layer is formed over the exposed portions. A second mask step selectively masks regions of the substrate for selective removal of the oxide layer. A p- or n-type dopant is then diffused into the exposed layers to form a portion of a switching element, such as a field-effect transistor (FET). Another mask and etch step, followed by a dopant step, completes the fabrication of the switching element. Further mask and etch steps are required to define the electrical connections for the circuit components and to passivate the circuit.
The use of poly-crystal silicon resistive elements within a photovoltaic generator circuit is problematic. In particular, resistors formed of poly-crystal silicon must be lightly doped with a dopant of an opposite conductivity type to that of the silicon. At the low dopant level required to provide sufficiently high impedance, the crystal lattice of poly-crystal silicon is unstable, and thus its performance over the expected operating temperature range of the device is also unstable. Also, as outlined above, the fabrication of an integrated circuit in which a lightly-doped, high-impedance poly-crystal silicon resistor is used is relatively complex and is thus relatively labor-intensive and costly.
It would therefore be advantageous to provide a photovoltaic generator circuit which can be fabricated in fewer process steps than those of the prior art and which overcomes the disadvantages of the prior art photovoltaic generator circuits.